1. Field of the Invention
The present invention relates to a thin-type optical device that guides display-image light to a viewer's pupil, and a virtual image display including the optical device and that displays a two-dimensional image as a virtual image enlarged by a virtual image optical system to the viewer.
2. Description of the Related Art
To display a two-dimensional image as a virtual image enlarged by a virtual image optical system to the viewer, there is well known a virtual image display, generally indicated with a reference numeral 100, as shown in FIG. 1. This virtual image display 100 uses a reflection-type volume hologram grating selected from among the hologram optical elements.
As shown in FIG. 1, the virtual image display 100 includes an image display element 111 to display an image, and a virtual image optical system to receive display-image light displayed on the image display element 111 and guide it to a viewer's pupil 116.
The virtual image display element 111 is, for example, an organic EL (electroluminescence) display, inorganic EL display, liquid crystal display (LCD) or the like.
As shown, the virtual image optical system includes a collimation optical system 121, optical waveguide 122 and first and second reflection-type volume hologram gratings 123 and 124 provided on the optical waveguide 122.
The collimation optical system 121 is to receive light beams emitted from pixels of the image display element 111 and form them into a group of parallel light beams different in angle of view from each other. The group of parallel light beams emitted from the collimation optical system 121 and different in angle of view from each other is incident upon the optical waveguide 122.
The optical waveguide 122 is a thin-type parallel flat one including, as main surfaces thereof, an optical surface 122c having a light inlet 122a provided at one end thereof to receive the group of parallel light beams coming from the collimation optical system 121 and different in angle of view from each other and a light outlet 122b provided at the other end to allow the light beams go out, and an optical surface 122d opposite to the optical surface 122c. 
The optical surface 122d of the optical waveguide 122 has the first reflection-type volume hologram grating 123 provided in a position opposite to the light inlet 122a of the optical surface 122c, and the second reflection-type volume hologram grating 124 provided in a position opposite to the light outlet 122b of the optical surface 122c. 
FIG. 2 is a sectional view of the second reflection-type volume hologram grating 124 having interference fringes recorded thereon. As shown in FIG. 2, the second reflection-type volume hologram grating 124 has groups of interference fringes, each group including three types of interference fringes 124a, 124b and 124c of, for example, different slant angles recorded side by side at the same pitch on a hologram surface 124S thereof. With the three types of interference fringes 124a, 124b and 124c different in slant angle from each other being recorded in the second reflection-type volume hologram grating 124, light beams to be diffracted are incident, at larger angles, upon the second reflection-type volume hologram grating 124. On the second reflection-type volume hologram grating 124, the three types of interference fringes 124a, 124b and 124c slanted at angles θa, θb and θc, respectively, are recorded at the same pitch, that is, at equal pitches irrespectively of their respective positions. The first reflection-type volume hologram grating 123 is shaped symmetrically with the second reflection-type volume hologram grating 124 with respect to a plane perpendicular to the optical surface. Further, the first and second reflection-type volume hologram gratings 123 and 124 are disposed on the optical surface 122d of the optical waveguide 122 for their interference fringes to be symmetric with each other with respect to a plane perpendicular to the optical surface 122d. 
The group of parallel light beams incident upon the light inlet 122a of the optical waveguide 122 and different in angle of view from each other are incident upon the first reflection-type volume hologram grating 123, and diffracted and reflected as they are. The diffracted and reflected group of parallel light beams will be propagated by repeated total reflection between the optical surfaces 122c and 122d of the optical waveguide 122 and be incident upon the second reflection-type volume hologram grating 124.
The optical waveguide 122 is designed to provide a light path having such a sufficient length and thickness (distance between the optical surfaces 122c and 122d) so that the group of parallel light beams different in angle of view and propagated by total reflection through the optical waveguide 122 will be subjected to different numbers of total reflections, respectively, depending upon their angles of view until they arrive at the second reflection-type volume hologram grating 124.
More specifically, parallel light beams of the group of parallel light beams incident upon the optical waveguide 122, that are incident being slanted toward the second reflection-type volume hologram grating 124, namely, those which are incident at large angles, will be reflected smaller numbers of times than those incident not being slanted so much toward the second reflection-type volume hologram grating 124, namely, those which are incident at small angles, because the group of parallel light beams incident upon the optical waveguide 122 are different in angle of view from each other. That is to say, since the parallel light beams of the group is incident upon the first reflection-type volume hologram grating 123 at different angles, respectively, so they are diffracted out at different angles, respectively, and thus are totally reflected at different angles, respectively. Therefore, the optical waveguides 122 may be designed sufficiently thin and long for the parallel light beams to be totally reflected different numbers of times.
The parallel light beams different in different angle of view from each other and incident upon the second reflection-type volume hologram grating 124 are diffracted and reflected so that they will not be subjected to total reflection, will be allowed to go out from the light outlet 122a of the optical waveguide 122 and be incident upon the viewer's pupil 116.
As above, the second reflection-type volume hologram grating 124 is disposed on the optical surface 122d of the optical waveguide 122 for interference fringes recorded thereon to be symmetric with those recorded on the first reflection-type volume hologram grating 123 with respect to a plane perpendicular to the optical surface. Therefore, since the group of parallel light beams reflected by the second reflection-type volume hologram grating 124 will be reflected at the same angles as angles of incidence upon the first reflection-type volume hologram grating 123, so an image will be displayed on the pupil 116 with a high resolution and without being blurred.
Including the first and second reflection-type volume hologram gratings 123 and 124 which will not work as any lens, the virtual image display 100 is able to display an image with less or no monochromatic eccentric aberration and diffraction color aberration.
However, the virtual image display 100 has incurred large unevenness of color and brightness in the past. That is, the slant angles of the interference fringes of the first and second reflection-type volume hologram gratings 123 and 124 of the virtual image display 100 are fixed in one hologram plane although the hologram layers are stacked together and slant interference fringes in each group are laid side by side in the hologram layer.
In this case, since the light beams different in angle of view from each other are incident upon the hologram at different angles so that the diffracted waves meeting the Bragg condition in different positions on the hologram are different in length from each other as shown in FIG. 3, light beams B91, B92 and B93 reflected in different positions will be diffracted with different efficiencies, respectively.
Namely, in case the light source wavelength spectrum incident upon the hologram as shown in FIG. 4 has a fixed band, the wavelength diffracted with a highest efficiency varies depending upon its angle of view, so the image will also vary in color depending upon a position in a display screen. That is, the wavelengths S91, S92 and S93 in FIG. 4 are diffracted with a highest efficiency in positions B91, B92 and B93, respectively, in FIG. 3. Therefore, in case the light source wavelength spectrum has a fixed band, the color will possibly be varied depending upon a position in a display screen. Also, in case the wavelength of light incident upon the hologram is short, the diffraction efficiency will possibly vary depending upon an angle of view and brightness will possibly be uneven.                Patent document 1: Published Japanese Translations of PCT International Publication for Patent Application No. 8-507879        Patent document 2: Japanese Patent Laid-Open No. 2002-162598        